Introduction:
The earliest reference to Krishna as a human hero can be found in the Chhandogya Upanishad. This ancient Hindu text is considered one of the principal Upanishads and is part of the Sama Veda. It contains philosophical and spiritual teachings that explore the nature of the self and the ultimate reality.

Explanation:
The Chhandogya Upanishad is divided into eight chapters, each dealing with different aspects of spiritual wisdom. In chapter three, known as the "Madhu Vidya" or "Honey Doctrine," Krishna is mentioned as a human hero in a dialogue between the sage Uddalaka and his son Shvetaketu.

Dialogue between Uddalaka and Shvetaketu:
The dialogue begins with Uddalaka asking his son if he has ever inquired about the nature of the Supreme Reality. Shvetaketu admits that he has not and requests his father to teach him. Uddalaka then proceeds to explain the concept of Brahman, the ultimate reality that underlies the entire universe.

In this context, Uddalaka narrates a story about a great warrior named Indra who once went to the gods seeking to know the nature of the self. The gods responded by telling him to meditate on the syllable "Om" (AUM), which represents the essence of Brahman.

Indra followed their advice and meditated on "Om" for a hundred years. After completing his meditation, he realized that "Om" was none other than the self within him. Uddalaka then equates Indra with Krishna, calling him a human hero who attained realization through his meditation on "Om."

Significance:
The mention of Krishna as a human hero in the Chhandogya Upanishad is significant because it highlights the idea that spiritual realization and the knowledge of Brahman are attainable by individuals through deep contemplation and meditation. Krishna, in this context, represents the epitome of a wise and enlightened being who has realized the ultimate truth.

This reference also foreshadows the later development of Krishna as a divine figure in Hindu mythology, where he is revered as an incarnation of Lord Vishnu and the central protagonist of the Bhagavad Gita.

Conclusion:
The Chhandogya Upanishad, specifically the dialogue between Uddalaka and Shvetaketu, provides one of the earliest references to Krishna as a human hero. This mention illustrates the significance of meditation and contemplation in attaining spiritual enlightenment and realizing the ultimate reality. The story of Indra, equated with Krishna, serves as a metaphor for the journey of self-discovery and the realization of the divine within oneself.